Exploring Dynamically Controlled Frisbee Throws Using a Highly Compliant Robotic Arm

When humans perform dynamic motions such as throwing, the passive properties such as the stiffness and damping of their arm is known to contribute to the task performance. By developing a robot arm which enables the stiffness of the different joints to be set programmatically, its contribution to the throwing behaviours can be determined. In addition to enabling new capabilities in robots this can also be useful for understanding how humans may perform such tasks. Utilizing permanent magnet synchronous motors (PMSM) and integrating them in back-drivable configurations we present a method of achieving programmable, precise, high bandwidth stiffness control. With a two joint variable stiffness arm, we experimentally explore the role of stiffness and coordination of actuation timings for the throwing of a Frisbee disk. From this exploration key trends between stiffness and the throwing distance and angle are observed. Considering variable stiffness (VS) we also see that the role and significance of VS varies depending on the overall energy levels of the system. For low energies, having a constant torque profile can enable a 30% increase in throwing distance, where as at higher energies VS is less significant. When compared to human throwers, the robot performs comparable to experienced humans for a short distance throwing task.